This invention relates to glass breakage detectors, and in particular to glass breakage detectors that utilize digital signal processing to determine if the signals produced by an acoustic transducer are the result of glass breakage. The term glass breakage as used herein refers to the breakage of framed glass, such as windows or doors, and not to the breakage of glass items, such as drinking glasses and the like.
Home and commercial security systems commonly use glass breakage detectors to detect the presence of an intruder. When an intruder breaks a window to enter the premises, the glass breakage detector detects the breakage of glass and an alarm is sounded. Glass breakage detectors with acoustic transducers monitor the sounds in the local environment. Acoustic glass breakage detectors of the prior art monitor the amplitude of the sound at frequencies that are typically associated with glass breakage to determine if the received sound is a result of glass breakage.
Acoustic detectors available today have a tendency to generate false alarms on other noises found in the home or business such as the shaking of keys, slamming of a file drawer, clapping of hands, etc. In order to reduce the incidence of false alarms, acoustic detectors of the prior art use multiple analog filters in order to selectively pass only frequencies associated with the breakage of glass. A glass breakage detector which comprises multiple hardware filters and which monitors the amplitude of the filtered signals is disclosed in U.S. Pat No. 5,323,141, which is incorporated by reference herein. The amplitudes within the chosen bands are compared to a predetermined threshold value in order to detect the glass breakage.
Another glass breakage detector of the prior art, disclosed in U.S. Pat No. 5,552,770, recognizes temporal events that typically accompany glass breakage. The high frequency sound of the impact is detected, followed by low frequencies caused by flexing of the glass due to the impact, and high frequencies again when the glass breaks by shattering. An alarm signal is issued by the glass breakage detector only when the detected low frequencies last for a predetermined minimum duration beginning not before the first detection of high frequencies. This glass breakage detector uses hardware filters and timing circuits to detect the glass breakage. Such a detector is an improvement over other acoustic detectors, but the improvement comes at the cost of extra hardware circuits. The size and cost of the hardware places limits on the number of filters a detector can have.
Acoustic detectors of today need adjustments during installation to work properly in the different environments in which they are installed. Acoustic waves resulting from a glass breakage event are a function of glass type, window frame configuration, room acoustics, and distance from the window. A small change in distance between the window and the transducer results in a large change in the received sound level. A range adjustment allows an installer to change the sensitivity of the acoustic detector to adapt it to its placement in the room. This adjustment may sometimes cause the detector to miss a glass breakage event. When the range setting is adjusted improperly by the installer, the breaking of a window may not exceed the detector""s threshold. To compound the problem, the installer remains unaware of the improper installation since a typical installation generally does not involve breaking an actual window. Some manufacturers design acoustic detectors with high gain amplifiers to ensure detection of glass breakage from the maximum recommended distance; this, however, results in amplifier saturation when the detector is mounted near the glass. It would be advantageous to have an acoustic detector which operates reliably over a vast range of sound levels thereby reducing installation errors.
In many environments, sounds specific to that environment create false alarms that are not easily discriminated against by acoustic detectors available today. In these environments, it would be advantageous to customize the detector by analyzing the sounds produced by the specific false alarm and modifying the detector to discriminate against that sound. It would also be advantageous to store the features of the sounds that generate an alarm so that later analysis of these features is possible.
It is therefore an object of the present invention to provide a glass breakage detection device with increased sensitivity without increased false alarms.
It is a further object of the present invention to provide a glass breakage detection device that detects a plurality of the features generated during a glass breakage event.
It is a further object of the present invention to provide a glass breakage detection device that may be adapted to detect a simulated glass breakage event during installation.
It is a further object of the present invention to provide a glass breakage detection device with the ability to be modified to include updated technology or to be customized for a particular environment.
It is a further object of the present invention to provide a glass breakage detection device that compensates for the characteristics of the room in which it is mounted.
It is a further object of the present invention to provide a device that corrects the front end offset errors of the glass breakage detection device.
It is a further object of the present invention to provide a device that transmits and stores features for computer analysis.
In accordance with these and other objects, the present invention is a method and a device for detecting the breakage of framed glass. The glass breakage detector comprises an acoustic transducer for sensing acoustic waves, an analog-to-digital (A/D) converter, and a processing means which uses software algorithms to extract features indicative of characteristics of the acoustic wave sensed by the acoustic transducer and analyze the extracted features to determine if the acoustic wave was a result of glass breaking. The acoustic transducer is adapted for a substantially flat gain response of the frequency range from approximately 20 Hz to approximately 20 kHz and the A/D converter samples the signal produced by the acoustic transducer at 44.1 kHz.
The glass breakage detector further comprises amplifiers for amplifying the analog signal from the acoustic transducer. The gain response of the amplifiers is greater for higher frequency components and approximately unity for lower frequency components. The offset error generated by the amplifiers may be corrected by the processing means before the signal is used for determining glass breakage. The processing means collects samples of the DC component of the amplified signal and samples of the amplified signal. To calculate the offset error, the processing means collects 1024 samples of both signals, subtracts the samples, and computes an average of the differences. The processor will subtract the computed average from future samples of the amplified signal to correct the offset error.
The processing means or digital signal processor (DSP) uses a feature extraction software algorithm that extracts features using a plurality of filters centered at different frequencies. The features include the summed energy, the period, the symmetry, and the number of zero crossings of the signal after it is filtered. Once the features are extracted, they are compared with stored values to determine if the sound is the result of a glass breakage by the rules analysis software algorithm. The processing means also uses an algorithm which distinguishes against difficult false alarms by checking the extracted features against characteristics of specific false alarms such as keys on a window. The processing means is also capable of transmitting the extracted features to an external computing device for further analysis.
An important feature of the present invention is the ability of the processing means to use different software routines which may be selected by a user for processing the signal from the acoustic transducer. A user can operate a switch to select a software algorithm from a number of sets of rules to analyze the extracted features to determine if the received waves are a result of glass breakage. This may be useful for reducing false alarms created by different environments. Similarly, a test mode switch causes the processing means to use a different software algorithm (that uses a 5 kHz filter) to extract features and a different rules analysis software algorithm to compare the extracted features against predetermined thresholds.
Another feature of the present invention is the ability of the factory to make changes to the software algorithm. Changes are made simply by reprogramming the algorithm stored in the processor""s memory. This feature allows the glass breakage detector to be easily updated with current technology without changing any of the hardware, thereby keeping it from becoming obsolete. This feature also allows the glass breakage detector to be customized to meet specific requirements of different environments.
Modifying or customizing the processing performed by the acoustic detector is accomplished by the following steps: generating a sound, sensing the sound with an acoustic transducer, processing the sound by digital conversion, extracting the features, transmitting the extracted features to an external computing device, analyzing the extracted features with the external computing device, determining a modification to the algorithm stored in memory, and modifying the algorithm.
Another aspect of the present invention is a processing device that can receive a signal from an acoustic transducer and process the signal using an algorithm stored in memory to determine if the signal is the result of glass breakage. The processing device may be located in a common housing with the acoustic transducer or may be located remotely from the transducer, receiving the signal by hardwired connection, optical transmission or radio frequency (RF) transmission. The device may also receive signals from a number of acoustic transducers, each having a unique identification number (ID). The signals from each acoustic transducer may be processed using the same algorithm or separate algorithms that correspond with the ID""s of the acoustic transducer.
The processing device may also have means for communicating to a control unit, a console, or a central station in order to receive commands. The commands include selecting different software algorithms from a set of predefined algorithms stored in memory to process the signal from the acoustic transducer. The commands may also modify a software algorithm stored in memory, or cause the processing device to transmit the extracted features stored in memory. The extracted features which may be from a historical event or a real time event may be transmitted to a central station via the communication means. This would allow the central station to monitor what has happened or what is presently happening in the environment that the acoustic detector is monitoring.